1. Field of the Invention
The present invention relates to a rewriteable optical disk and a method of manufacturing the rewriteable optical disk. More particularly, the present invention relates to a rewriteable optical disk, and method of manufacturing the optical disk, which provides a recording film having uniform film thickness in a land portion and in a groove portion.
2. Description of the Related Art
Recently, there has been a demand for rewriteable optical disks having higher recording density to provide an optical disk having increased capacity. The land/groove recording method has been proposed to increase the recording density of the rewriteable optical disk. In the conventional land/groove recording method, information is recorded on the optical disk in both a bottom portion (groove portion) of grooves formed in concentric circles in the optical disk, and in a portion between adjacent grooves (land portion).
More particularly, in the land/groove recording method, the track pitch (which corresponds to the pitch between an adjacent land portion and groove portion) is about half of the track pitch produced by the method of recording on only the lands or on only the grooves, thereby increasing the recording density (track density) in the track width direction.
However, in optical disks using the land/groove recording method, for example, heat occurs when light is irradiated on the land portions during the recording of information on the land portions or during erasure of information recorded on the land portions. The heat which occurs when light is irradiated on the land portions is transmitted to the adjacent groove portions, and information recorded on the groove portion is partially erased (referred to hereinbelow as cross-erasure) as a result of the temperature rise at this time.
Japanese Laid-Open Patent Applications Nos. JP-A-9-161321 and JP-A-9-251674 disclose a disk which solves the problem of cross-erasure by making the transmission of heat between the land portion and the groove portion difficult. In an optical disk produced by the land/groove recording method, the transmission of heat between the land portion and groove portion is made difficult by making the difference in level between adjacent land portions and groove portions large, thus making the groove depth deep, and by making the distance between the land portions and groove portions large.
However, because the groove depth is made deep to avoid cross-erasure in the known optical disk using the land/groove recording method, problems such as the following occur during film formation of a recording film.
A magnetron sputtering method is generally used to perform the film formation of the recording film on the substrate of an optical disk. As shown in FIG. 5, in the magnetron sputtering method, particles 3 fly from a target 4 according to a cosine law in a direction along a line L4 normal to the target 4, as shown by the direction of the arrow 31, and in various directions at inclinations from the normal line L4, as shown by the directions of the arrows 32, 33, 34 and 35.
Among the particles 3 which fly from the target 4, the particles 3 which fly in the direction along the line L4 normal to the target 4 in the direction of the arrow 31 are incident about perpendicularly on the substrate 1, and can reach the land portions 2c and groove portions 2d of the groove 2 in the same way.
On the other hand, the particles 3 which fly out in the directions of arrows 32, 33, 34, 35, inclined from the normal line L4 of the target 4, are incident at an inclination to the substrate 1, and adhere to the sidewalls 2a, 2b of the grooves 2 and, screened by the land portions 2c, adhere to the land portions 2c but do not reach the groove portions 2d. Among the particles 3 which fly out at an inclination to the substrate 1 in the directions of the arrows 32, 33, 34, 35, the proportion of particles actually reaching the groove portion 2d becomes lower, the deeper the depth D of the grooves 2 (deep groove optical disk).
In the above-described manner, in a deep groove optical disk, the proportion of the particles 3 adhering to the groove portion 2d is markedly low. As a result, the recording film becomes thick at the land portions 2c, and becomes thin at the groove portions 2d, and the difference in film thickness between the land portions 2c and groove portions 2d of film which has been formed becomes markedly large. However, the laser power required when performing land/groove recording on an optical disk differs according to the film thickness of the recording film.
Optimum recording of information cannot be performed with an optical disk having a thick film thickness of the land portions 2c, and a thin film thickness of the groove portions 2d, as described above, if the laser power when recording is performed on the land portions 2c or the groove portions 2d is suitable for a different portion. More particularly, if the laser power is at a value which is suitable for recording or erasure of information on the groove portion 2d, the laser power is insufficient in the land portions 2c to the extent that the film thickness is thick, and optimum recording or erasure of information cannot be performed. Moreover, if the laser power when performing recording on land portions 2c and groove portions 2d is set at a value which is suitable for recording or erasure of information at the land portions 2c, the laser power in the groove portions 2d is excessive to the extent that the film thickness is thin, and optimum recording or erasure of information cannot be performed.
Accordingly, for the conventional optical disk described above, when the land portions 2c of the recording film are thick and the groove portions 2d of the recording film are thin, the laser power to perform recording or erasure of information in the land portions 2c has to be at a value different from the laser power to perform recording or erasure of information in the groove portions 2d. When the groove portions 2d of the recording film are thin in comparison with the land portions 2c, the laser power to perform recording or erasure of information in the groove portions 2d is set to a lower value than for the land portions 2c. In the above manner, when different values of the laser power are set to perform recording or erasure of information in the land portions 2c and groove portions 2d, the structure and operation of the recording and playback device becomes complicated.
Furthermore, when using a recording and playback device having the complicated structure and operation, even when the recording or erasure of information in the land portions 2c and the groove portions 2d is performed at different laser powers, a problem occurs in that the information in the groove portions 2d becomes partially erased. More particularly, the information in the groove portions 2d becomes partially erased because the heat generated by the light having high power irradiated when recording or erasing the information in the land portions 2c becomes transmitted to the groove portions 2d, in spite of the groove 2 having been deepened Further, because the groove portion 2d having thin film thickness has a small thermal capacity, the groove portion 2d quickly reaches a high temperature.
Accordingly, cross-erasure in the groove portion 2d of an optical disk using the conventional land/groove recording method is not avoided, even through the depth of the groove 2 is increased to avoid cross-erasure in the conventional land/groove recording method, because the film thickness of the land portion 2c is thick and the film thickness of the groove portion 2d is thin.